EP3819985B1

EP3819985B1 - Microstrip patch antenna with increased bandwidth

Info

Publication number: EP3819985B1
Application number: EP19208147.9A
Authority: EP
Inventors: Marat Patotski
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2019-11-08
Filing date: 2019-11-08
Publication date: 2024-04-24
Anticipated expiration: 2039-11-08
Also published as: US11495891B2; US11837791B2; CN112787108A; EP3819985A1; US20230026995A1; US20210143557A1

Description

The present disclosure relates to microwave antennas, particularly to microstrip patch antenna arrays.
High frequency radio transmission and microwave transmission, particularly in the 1 to 10 GHz range, is of great importance to high-speed data transmissions having low power consumption. Additionally, the increasing density of components on printed circuit boards (PCBs) calls for advances that reduce the size of individual components on the PCB to facilitate further component density increases.
Microstrip patch antennas are becoming increasingly useful as they can be printed directly onto a circuit board and their low profile and small size suits them particularly to applications where parameters such as space and weight is at a premium. Existing patch antennas are typically low cost and are easily fabricated.
WO 2017/150054 A1 discloses a microstrip patch antenna having a plurality of feeding elements and a plurality of parasitic elements, wherein these elements are arranged such that each feeding element makes use of two parasitic elements on opposite sides of the feeding element, and parasitic elements positioned between two feeding elements are shared by both feeding elements.
JP 2009 089217 A discloses an antenna array formed from radiating elements and parasitic elements and is directed to a problem of providing an antenna array having high gain while suppressing the formation of grating lobes.
US 2015/236408 A1 discloses a reconfigurable radio frequency aperture, comprising a substrate, a plurality of reconfigurable patches on the substrate, and a plurality of reconfigurable coupling elements on the substrate, wherein at least one reconfigurable coupling element is coupled between two reconfigurable patches.
Any one of the publications by Zou Xiaojun ET AL: "Microstrip Antenna Array of Connected Elements Using X-Shaped Connection Line", IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 17, NO. 5, 1 May 2018 (2018-05-01), pages 890-893 and GARCIA R ET AL: "Coherently radiating periodic structures (CORPS): a step towards high resolution imaging systems?", ANTENNAS AND PROPAGATION SOCIETY SYMPOSIUM, 2005, IEEE WASHINGTON, DC, JULY 3-8, 2005, PISCATAWAY, NJ : IEEE, US, vol. 4B, 3 July 2005 (2005-07-03), pages 347-350, DOI: 10.1109/APS.2005.1552819, ISBN: 978-0-7803-8883-3 discloses an antenna array of at least three actively fed patches, wherein parasitic patches are only disposed between the active patches.
Viewed from a first aspect, the invention provides a microstrip antenna array as defined in claim 1.
An advantage of the first aspect is to increase the bandwidth of the patch antenna array by around 50% or more, depending on the particular materials and construction of the patch used. Also, the use of a thin substrate has an advantage of increased structural flexibility and reduced manufacturing costs.
A microstrip is a type of transmission line that may be used for the transmission of microwave, terahertz, or high frequency radio waves. Microstrip structures may be fabricated on printed circuit board (PCB) or as part of monolithic microwave integrated circuits (MMICs) using conventional methods known to the skilled person. Such methods include, but are not limited to, milling, screen printing, and chemical etching. Thus, the microstrip patch antenna may be formed on a PCB by one of those techniques.
A substrate may be considered to be "thin" when the substrate is significantly smaller in thickness in comparison to the wavelength of the frequency of the antenna on the substrate, specifically in relation to the wavelength of the antenna in the dielectric substrate λ_d . This wavelength is modified from the wavelength of the signal in free space λ ₀ by the relative dielectric constant of the substrate material ε_r , where $λ_{d} \propto {ε_{r}}^{- \frac{1}{2}}$
. Thus, media with higher dielectric constants would result in a shorter signal wavelength in the dielectric. The thin substrate may comprise a single layer of substrate material, where the material may have a thickness of around 1.0 mm or less, such as 0.5 mm, 0.2 mm or 0.1 mm. Substrate materials such as Duroid, Teflon or FR4 may be suitable for thin film patch antennas. Thin substrates may be more flexible than thicker single layer substrates or multilayer substrates. The use of a thin substrate for a patch antenna array may allow the array to be formed around rounded objects or fit into spaces that would otherwise be difficult for arrays using thicker substrates to conform to.
Microstrip structures may be formed on the conducting layer of a PCB, which is the layer of conducting material on top of the PCB substrate. The conducting layer may be relatively thin compared to the thickness of the substrate. The shape of a microstrip structure may be two-dimensional in the plane of the conducting layer and the structure may be formed by etching or milling the conducting layer of a PCB to remove unwanted conducting material. Each microstrip structure in the conducting plane may have a uniform thickness.
The ground layer is on the opposite side of the substrate to the conducting layer. The ground layer may be uniform in thickness and may be formed from the same material as the conducting layer. The ground layer may be defectless or may have defects formed in its surface. The ground layer may cover all of the substrate on the side on which it is placed.
A parasitic element or passive radiator is a conductive element which is not electrically connected to any other component. In other words, parasitic components do not have an input port and are not driven directly.
The microwave patch antenna comprises more than two radiating patches formed on a substrate. The structure as a whole, including the more than two radiating patches, may collectively be referred to as an "array". The radiating patches may be formed in single row on a substrate. Each radiating patch may be oriented in the same direction on the same substrate. Each radiating patch may be equally spaced along the common longitudinal axis in the longitudinal direction of the substrate. Each radiating patch may be regularly spaced along the common longitudinal axis such that the radiating transverse axis of each adjacent radiating patch is equidistance from one another. The distance between two adjacent radiating patches may be about 0.5λ_o , or may be in the range of 0.25_λo to 0.75λ_o . Alternatively, the distance between two radiating patches in an array of more than two radiating patches may not be regular.
Each radiating patch may have equal dimensions, that is, the radiating patch widths and the radiating patch lengths of each radiating patch are the same. Alternatively, radiating patches may have radiating patch widths and/or radiating patch lengths that differ between individual radiating patches or subsets of patches.
A VIA is an electrical connection between the conducting metal on one side of the substrate and the ground plane on the other side of the substrate and may be a through hole where the edges of the hole are coated in a conducting material.
One or more VIAs are placed along the parasitic patch longitudinal axis and divide the conducting metal portion of the parasitic patch into two quarter wavelength λ_d /4 resonant portions. The quarter wavelength λ_d /4 portions may be coupled together through the one or more VIAs. This coupling may create an additional resonance frequency f ₃. In the case of two or more VIAs, the distance between VIAs and the diameters of the VIAs is tuned to provide necessary coupling between two quarter wavelength λ_d /4 resonance portions. VIAs may be positioned to form resonant portions of other lengths.
The parasitic patch structure may have a total parasitic patch width and a total parasitic patch length, wherein these dimensions may encompass all components in a parasitic patch in the conducting plane. These total lengths may include additional features of the parasitic patch, such as VIAs, or may cover the extent of a patch that is formed from more than one parasitic microstrip line. The parasitic patch may not be in physical contact with any of the radiating patches in the conducting plane.
At least one of the more than one parasitic patch may be symmetric about the common longitudinal axis.
At least one of the more than one parasitic patch may be symmetric about its parasitic patch transverse axis.
At least one of the more than two radiating patches may be symmetric about its radiating patch transverse axis.
The microstrip array of the first aspect may use a microstrip feed, which is the excitation of the microstrip antenna by a microstrip line on the same conducting layer. A microwave patch antenna may alternatively be fed in a number of other non-limiting ways, such as: directly at the end of the patch; using an inset feed; using a quarter-wave impedance matching transmission line; from underneath using a coaxial cable or probe feed; using coupled feeds; or using aperture feeds. The particular type of feed may be dependent upon the particular application of the patch antenna, and is not limited to those mentioned here. Any feedline may be connected to the input port of the radiating patches.
Certain embodiments of the disclosure will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1A is a top view of a prior art microstrip antenna array.
Figure 1B is a side view of a prior art microstrip antenna array.
Figure 2A is a top view of an example microstrip antenna array not falling under the scope of the appended claims.
Figure 2B is a side view of an example microstrip antenna array not falling under the scope of the appended claims.
Figure 3A is a top view of an embodiment of a microstrip antenna array according to the invention.
Figure 3B is a side view of an embodiment of a microstrip antenna array according to the invention.
Figure 4A is a top view of yet another example microstrip antenna array not falling under the scope of the appended claims.
Figure 4B is a side view of yet another example microstrip antenna array not falling under the scope of the appended claims.
Figure 5 shows S-parameters for the prior art antenna array and for each of the example antenna arrays and the embodiment of an antenna array according to the invention.
Figure 6 shows a graph of the voltage standing wave ratio (VSWR) at the input of a radiating patch for each of the prior art and example antenna arrays and the embodiment of an antenna array according to the invention.
Figure 7 shows a spherical polar coordinate system applied to a microstrip antenna array.
Figures 8A and 8B are radiation patterns of the prior art patch antenna array and for each of the example arrays and the embodiment of an antenna array according to the invention at angles of ϕ = 0 and ϕ = 90 based upon the coordinate system shown in Figure 7.

A prior art patch antenna array 100 is presented in Figure 1A and 1B, where Fig. 1A shows a top view of the array 100 to display an arrangement of radiating patches 102 and Fig. 1B shows a side view of the array 100. The prior art patch antenna array 100 comprises a substrate 104 formed from a single layer of substrate material, a layer of conducting material forming a ground layer 106 on the bottom side of the substrate 104, and a plurality of radiating patches 102 on a top side 108 of the substrate 104. Each radiating patch 102 has an input port 110, a radiating patch width W_RP extending in a longitudinal direction, and a radiating patch length L_RP extending in a transverse direction. The each of the plurality of radiating patches 102 are spaced along a common longitudinal axis C and are oriented so that the input ports 110 for each of the radiating patches 102 are oriented in the same direction.
Each radiating patch 102 also comprises a radiating patch transverse axis T along the midpoint of the radiating patch width W_RP. Starting from the leftmost radiating patch in Fig. 1A and moving rightwards, the radiating patches 102 may be labelled RP1, RP2 ... RPN, for N number of radiating patches 102. The distance between the transverse axes T of two adjacent patches 102, starting from the distance between RP1 and RP2 and moving rightwards may then be labelled S_RP1, S_RP2 ... S_RP(N-1).
The mutual coupling between patches 102 is characterized either by the conductance matrix (G-matrix) or by the scattering matrix (S-matrix).
The mutual conductance between two rectangular microstrip patches for the radiating patch arrangement is [1]: $G_{12} = \frac{2}{π} \cdot \sqrt{\frac{ε}{μ}} \cdot \int_{0}^{π} {[\frac{\sin (\frac{k_{0} W}{2} \cdot \cos θ)}{\cos θ}]}^{2} \sin^{3} θ \cdot \cos (\frac{Z}{λ}) (_{0} 2 π \cdot \cos θ) [1 + J_{0} (\frac{L}{λ} 2 π \sin θ)] dθ$

J ₀ - the Bessel function of the first kind of order zero;
Z - the center-to-center separation between the patches and equal to the array step S_RP;
W - the width of the radiating patch;
L - the length of the radiating patch;
λ ₀ - is the wavelength in free space;
ε - the permittivity of free space;
µ - the permeability of free space.

In the prior art array 100 shown in Figures 1A and 1B, the fields in the space between the elements are primarily transverse electric (TE) modes and there is not a strong dominant mode surface wave excitation. Therefore, there is reduced coupling between the elements. When the coupling is small, the resonant frequency of the patch radiator is close to the resonant frequency of uncoupled antennas f ₀.
When the strength of coupling increases, two resonant frequencies f ₁ and f ₂ of coupled patches appear. The strength of coupling is described with the coupling coefficient k that can be computed from the following formula: $k = \frac{f_{2}^{2} - f_{1}^{2}}{f_{2}^{2} + f_{1}^{2}}$

f₁ - the lower resonant frequency of coupled antennas;
f ₂ - the upper resonant frequency of coupled antennas.

To improve the coupling between radiating patches a parasitic patch is used. Placing a resonance structure (the parasitic patch) between active radiating patches increases coupling between the radiating patches and provides mutual detuning of radiators. Active radiating patches are radiating patches that are being fed with a signal via the input port of the radiating patch.
One example of a microstrip patch antenna array 200 having parasitic patches is shown in Figures 2A and 2B. The microstrip antenna array 200 comprises a thin substrate 204 and more than two microstrip radiating patches 202 placed on a first side 208 of the substrate 204. Each radiating patch 202 comprises an input port 210, a radiating patch width W_RP extending in a longitudinal direction, and a radiating patch length L_RP extending in a transverse direction, wherein the transverse direction is perpendicular to the longitudinal direction, and wherein the longitudinal and transverse directions are in the plane of the radiating patch 202. Each patch 202 also comprises a radiating patch transverse axis T_RP along the midpoint of the radiating patch width W_RP and a radiating patch longitudinal axis along the midpoint of the radiating patch length. The more than two radiating patches 202 are spaced in the longitudinal direction such that the radiating patch longitudinal axis of each radiating patch 202 is aligned along a common longitudinal axis C.
The microstrip patch array 200 also comprises more than one parasitic patches 212 placed on the first side 208 of the substrate 204, wherein there are at least one fewer parasitic patches 212 than there are radiating patches 202. Each parasitic patch 212 comprises a parasitic patch width W_PP extending in the longitudinal direction, a parasitic patch length L_PP extending in the transverse direction, a parasitic patch transverse axis T_PP along the midpoint of the parasitic patch width, and a parasitic patch longitudinal axis along the midpoint of the parasitic patch length. The more than one parasitic patches 212 are spaced in the longitudinal direction such that the parasitic patch longitudinal axis of each parasitic patch 212 is aligned along the common longitudinal axis C.
Each parasitic patch 212 is positioned between two radiating patches 202 and the parasitic patch transverse axis T_PP of each parasitic patch is positioned at the midpoint between the radiating patch transverse axes T_RP of the two radiating patches 202 either side of each parasitic patch 212.
The parasitic patch 212 has such dimensions so that to provide necessary coupling k between radiating patches 202. The length of parasitic patch L_PP is approximately close to a half wavelength in substrate λ_d at a central working frequency f ₀. The parasitic patch width W_PP and gaps between parasitic patches and radiating patches G_P are tuned to provide the certain strength of coupling k between radiating patches 202.
An embodiment of a microstrip patch antenna array 300 according to the invention is shown in Figures 3A and 3B. The construction of the antenna array 300 is similar to that of the previous example in that the radiating patches 302 are the same and the parasitic patches 312 comprise a strip of conducting metal, each parasitic patch 312 being positioned between two radiating patches 302. That is, the length of parasitic patch L_PP approximately is close to half wavelength in substrate λ_d /2 at central working frequency f ₀. The width of parasitic patch W_PP and gaps between parasitic patches and radiating patches G_P are tuned to provide the certain strength of coupling k between radiating patches.
The parasitic patches 312 shown in Figures 3A and 3B also comprise two VIAs 314 in each patch 312. The VIAs 314 are an electrical connection between the conducting metal portion of the parasitic patch 312 and the ground plane, passing though the substrate. The VIAs 314 are positioned within the area of the conducting metal portion of the parasitic patch 312 and along the common longitudinal axis C. The VIAs 314 are placed along the parasitic patch longitudinal axis and divide the conducting metal portion of the parasitic patch 312 into two quarter wavelength λ_d /4 resonant portions 316. The quarter wavelength λ_d /4 portions 316 are coupled together through the VIAs 314. This coupling creates an additional resonance frequency f ₃. The distance between VIAs 314 and their diameters is tuned to provide necessary coupling between the two quarter wavelength λ_d /4 resonance portions 316.
Yet another example of a microstrip patch antenna array 400 is shown in Figures 4A and 4B. In this example, the radiating patches 402 are the same as in the previous two examples. The parasitic patch 412 in this example comprises two parasitic microstrip lines 414 are placed between the radiating patches 402. The length of parasitic microstrip lines L_PML approximately is close to a half wavelength of the signal in substrate λ_d /2 at the central working frequency f ₀. Each parasitic microstrip line has a width W_PML. The gaps between parasitic microstrip lines and radiating patches G_P are tuned to provide the certain strength of coupling k between radiating patches 402. The parasitic microstrip lines 414 are coupled together through the gap G_PML. This coupling creates an additional resonance frequency f ₃. The gap between parasitic microstrip lines G_PML is tuned to provide necessary coupling between them.
The S-parameters for the prior art antenna array and for each of the examples and the embodiment according to the invention are shown in Figure 5. S-parameters characterize the mutual coupling between radiating patches, and the S₂₁ parameter indicates power loss or gain at the output of the system as compared to the energy put into the system.
Figure 6 shows a graph of the voltage standing wave ratio (VSWR) at the input of a radiating patch for each of the prior art and the above example antenna arrays and the embodiment of an antenna array according to the invention. At a VSWR of, 10% of the input power is reflected and this is a level at which the antenna may be considered to be impedance matched with the input feedline. At this value, it can be clearly seen from the graph that the bandwidth for each of the example patch arrays is significantly wider than that of the prior art array.
Figure 7 shows a spherical polar coordinate system, where the x-axis is collinear with the common longitudinal axis, the y-axis is parallel to the transverse direction, and the z-axis is in a direction upwards from the substrate and antenna and is perpendicular to the conducting plane. The origin of the coordinate axis is at the midpoint between two radiating patches.
Figures 8A and 8B are radiation patterns of the prior art patch antenna array and for each of the example arrays and the embodiment of an antenna array according to the invention at angles of ϕ = 0 and ϕ = 90 based upon the coordinate system shown in Figure 7. The mutual coupling between the radiating patches and the parasitic patches causes a slight distortion of the radiating characteristic of radiating patch G(θ) and reduces the gain of the radiating patch no higher than 1.5 dB, which is appropriate for many applications.

Claims

A microstrip antenna array (200; 300; 400) comprising:
a thin substrate (204);

more than two microstrip radiating patches (202; 302; 402) placed on a first side (208) of the substrate (204), each radiating patch (202; 302; 402) comprising:
an input port (210);

a radiating patch width (W_RP) extending in a longitudinal direction;

a radiating patch length (L_RP) extending in a transverse direction, wherein the transverse direction is perpendicular to the longitudinal direction, and wherein the longitudinal and transverse directions are in the plane of the radiating patch;

a radiating patch transverse axis (T_RP), the radiating patch transverse axis extending in the transverse direction and coinciding with the midpoint of the radiating patch width; and

a radiating patch longitudinal axis, the radiating patch longitudinal axis extending in the longitudinal direction and coinciding with the midpoint of the radiating patch length,

wherein the more than two radiating patches are spaced in the longitudinal direction such that the radiating patch longitudinal axis of each radiating patch is aligned along a common longitudinal axis (C); and

more than one parasitic patch (212; 312; 412) placed on the first side (208) of the substrate (204), wherein there is at least one fewer parasitic patches than there are radiating patches, each parasitic patch comprising:
a parasitic patch width (W_PP) extending in the longitudinal direction;

a parasitic patch length (L_PP) extending in the transverse direction;

a parasitic patch transverse axis (T_PP), the parasitic patch transverse axis extending in the transverse direction and coinciding with the midpoint of the parasitic patch width; and

a parasitic patch longitudinal axis, the parasitic patch longitudinal axis extending in the longitudinal direction and coinciding with the midpoint of the parasitic patch length,

wherein the more than one parasitic patch (212; 312; 412) are spaced in the longitudinal direction such that the parasitic patch longitudinal axis of each parasitic patch is aligned along the common longitudinal axis (C), wherein each parasitic patch is positioned between two radiating patches (202; 302; 402), and wherein the parasitic patch transverse axis (T_PP) of each parasitic patch is coincident with the midpoint between the radiating patch transverse axes (T_RP) of the two radiating patches either side of each parasitic patch,

wherein the parasitic patch width and gaps between parasitic patches and radiating patches G_P are tuned to provide the certain strength of coupling k between radiating patches, and

characterised in that at least one parasitic patch comprises at least one VIA.
The array of claim 1, wherein the radiating patch input ports are positioned along the radiating patch transverse axis.
The array of claims 1 or 2, wherein the substrate has a thickness of 1.0mm or less.
The array of claims 1, 2 or 3, wherein the radiating patches are regularly spaced along the common longitudinal axis.
The array of any preceding claims, wherein the radiating patch transverse axes of adjacent radiating patches are separated by about a half wavelength of an input signal having a central working frequency ƒ ₀, wherein the wavelength of the signal is modified by the substrate.
The array of any preceding claim, wherein the parasitic patch length is about a half wavelength of an input signal having a central working frequency ƒ ₀, wherein the wavelength of the signal is modified by the substrate.
The array of any preceding claim, wherein at least one of the more than one parasitic patch is symmetric about the common longitudinal axis.
The array of any preceding claim, wherein at least one of the more than one parasitic patch is symmetric about its parasitic patch transverse axis.
The array of any preceding claim, wherein at least one of the more than two radiating patches is symmetric about its radiating patch transverse axis.
The array of any preceding claim, wherein the at least one VIA is positioned along the common longitudinal axis.
The array of any preceding claim, wherein the at least one VIA is positioned to divide the parasitic patch into two quarter wavelength λ_d /4 resonant portions at a central working frequency ƒ ₀.